Electronic messaging, particularly electronic mail (“e-mail”) carried over the Internet, is rapidly becoming not only quite pervasive in society but also, given its informality, ease of use and low cost, a preferred method of communication for many individuals and organizations.
Unfortunately, e-mail recipients are increasingly being subjected to unsolicited mass mailings, often referred to as spam. Most spam is commercial advertising, often for dubious products, get-rich-quick schemes, or quasi-legal services. The phenomenon of spam has become one of the Internet's biggest blemishes. With the growth of Internet-based commerce, a wide and growing variety of electronic merchandisers are repeatedly sending unsolicited mail advertising their products and services to an ever-expanding universe of e-mail recipients. Some consumers who order products or otherwise transact with a merchant over the Internet want to and, in fact, do regularly receive such solicitations from those merchants.
However, electronic mailers are continually expanding their distribution lists to reach an increasing number of recipients. For example, recipients who merely provide their e-mail addresses in response to perhaps innocuous appearing requests for visitor information generated by various web sites often receive unsolicited mail and much to their displeasure, they find that they have been included on electronic distribution lists. This occurs without the knowledge, let alone the assent, of the recipients. Some electronic mailers are now sending e-mail containing images that automatically invoke a link to a website that tracks the e-mail addresses of users who open the e-mail. The e-mail addresses are then added to the electronic mailer's distribution list. Furthermore, an electronic mailer will often disseminate its distribution list, whether by sale, lease or otherwise, to another such mailer for its use, and so forth with subsequent mailers. Consequently, over time, e-mail recipients often find themselves increasingly barraged by unsolicited mail resulting from separate distribution lists maintained by a wide and increasing variety of mass mailers. An individual can easily receive hundreds, and even thousands, of pieces of unsolicited e-mail over the course of a year. Individuals on e-distribution lists can expect to receive a considerably larger number of unsolicited messages over a much shorter period of time. Consumer groups, privacy advocates and Internet service providers have vocally rallied against spammers, claiming the email overloads people's in-boxes and companies' ISP networks.
Furthermore, while many unsolicited e-mail messages are benign, such as offers for discount office or computer supplies, mortgage rate quotes, or invitations to attend conferences of one type or another, others, such as pornographic, inflammatory and abusive material, are offensive to their recipients. These unsolicited messages are known as “junk” mail or as “spam.” The e-mail load from spam can be equivalent to the load generated from legitimate e-mail. Recent indications by one of the biggest ISPs indicate that of the estimated 30 million email messages each day, about 30% on average is unsolicited commercial email. Trends indicate that users will receive more and more spam. For example, it has been estimated that in 1999, the average consumer received 40 pieces of spam. By 2005, it has been estimated that the total is likely to soar to 2000 pieces of spam.
Similar to the task of handling junk postal mail, an e-mail recipient must sift through his incoming mail to remove the spam. The computer industry recognized this problem and has developed techniques to automate the removal of spam. For example, one technique is turf lists. E-mail recipients subscribe to turf lists, which identifies and refuses to accept mail using a defined rule based set of characteristics. Unfortunately, the choice of whether a given e-mail message is spam or not is highly dependent on the particular recipient and the actual content of the message. What may be spam to one recipient may not be spam to another, which limits the functionality of turf lists. Additionally, an electronic mailer (i.e., a spam generator) will prepare a message such that its true content is not apparent from its subject line and can only be discerned from reading the body of the message. Another technique developed is known as a black hole list. The black hole list is a list of known spam addresses from which spam is sent. The e-mail sender's address is checked against the black hole list. If the address is on the list, the e-mail is not accepted. Spam generators simply change their address to bypass this technique.
Other techniques have also been developed. A majority, if not all of these techniques are performed on the client side, which means that the spam still clogs up e-mail servers that store the spam and then deliver the spam to the client where it gets deleted. This wastes storage space and reduces the bandwidth available for legitimate uses. One technique checks to see if the sender of the e-mail is on an authorized list. If the sender is not on an authorized list, the e-mail is stored in a temporary spot and a message that only humans can read is embedded in a graphic image and the sender must reply to the message. If the sender replies, the e-mail is sent to the recipient and the sender's e-mail address is added to the authorized list. Another technique is to send the sender a message that the sender must place in a reply message. For example, the sender is directed to a website to enter a few letters the sender sees for the e-mail to be sent to the recipient. None of the techniques are 100% effective. For example, spammers hire people to respond to messages that only humans can read.